Leucite is a crystalline potassium aluminosilicate which, in stable form, possesses a tetragonal configuration at room temperature. Tetragonal leucite, also known as "low leucite", has been employed as a reinforcing agent in feldspathic dental porcelains. Such dental porcelain materials are described in, for example, U.S. Pat. Nos. 4,604,366 and 4,798,536. Since tetragonal leucite possesses a high coefficient of thermal expansion, the resulting feldspathic porcelains comprising tetragonal leucite dispersed therein as a discontinuous phase have correspondingly high coefficients of thermal expansion. For example, the tetragonal leucite-containing feldspathic porcelain powder sold under the trademark Optec.TM. by Jeneric/Pentron Inc., Wallingford, Conn. can be used to provide a dental porcelain body possessing a coefficient of thermal expansion of about 18.6.times.10.sup.-6 /.degree. C. when measured at 50 to 550.degree. C.
When tetragonal leucite is heated to about 625.degree. C., it changes to a cubic polymorph and exhibits a volume change of 1.2%. This transformation is reversible. Upon cooling, the cubic leucite crystals revert to the more stable tetragonal polymorph. In contrast to tetragonal leucite, the stabilized cubic phase of leucite, known as "high leucite", which is otherwise unstable at room temperature, possesses a coefficient of thermal expansion of about 3.times.10.sup.-6 /.degree. C. when measured at 625 to 900.degree. C.
Rouf et al. "Crystallization of Glasses in the Primary Field of Leucite in the K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 System", Trans. J. Brit. Ceram. Soc., 77:36-39 (1978) describe an isothermal heat treatment method of crystallizing cubic leucite in the high viscosity system of K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 for both powder and bulk samples which employs TiO.sub.2, ZrO.sub.2 and P.sub.2 O.sub.5 as catalysts. The Rouf et al. method employs the use of high temperatures and long time periods and relies on the presence of large amounts of K.sub.2 O (approximately 18 weight percent) in the starting glass composition to form cubic leucite as the only crystalline phase. Bulk samples of the porcelain produced by the method disclosed in Rouf et al. do not comprise cubic leucite substantially uniformly dispersed therein.
Hermansson et al. "On the Crystallization of the Glassy Phase in Whitewares," Trans. J. Brit. Ceram. Soc. 77:32-35 (1978), similarly disclose a heat treatment method of crystallizing cubic leucite in the high viscosity system of K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2. Hermansson et al. disclose that high K.sub.2 O content, a long firing time and a low content of CaO (approximately 1 weight percent) are required to stabilize the cubic phase of leucite at room temperature.
Prasad et al. "Crystallization of Cubic Leucite By Composition Additives", 19th Annual Session, American Association For Dental Research, (1990), describe a bulk crystallization method for stabilizing cubic leucite at room temperature by the addition of cesium oxide to a feldspathic composition. The method involves heating a raw material mix including cesium oxide to 1550.degree. C. for eight (8) hours, rapidly cooling the melt to 1025.degree. C. holding the resulting material for 1-4 hours isothermally to effect bulk crystallization of cubic leucite crystals followed by bench cooling the composition in air. The compositions disclosed in Prasad et al. are composed of mixtures of cubic leucite and tetragonal leucite and are highly refractory materials that can only be fused at temperatures in excess of 1200.degree. C.
None of the aforementioned prior art methods disclose an ion exchange step.
It is well-known in the art that it is extremely important that the thermal expansion coefficient of a dental porcelain closely match the thermal expansion coefficient of the metal or porcelain material with which it is in contact. Since the leucite-containing dental porcelains of the prior art generally possess high coefficients of thermal expansion, they cannot be employed in combination with materials possessing significantly lower thermal expansion coefficients. For example, low expansion porcelain veneering materials, such as SlipperyGlass.RTM. porcelain sold by Jeneric/Pentron, Incorporated (Wallingford, Conn.), cannot be employed with the high expansion leucite-containing dental porcelains of the prior art, such as Optec.TM. porcelain sold by Jeneric/Pentron, Incorporated (Wallingford, Conn.).
Accordingly, it is an object of the present invention to provide a low expansion feldspathic porcelain and a method whereby a low expansion feldspathic porcelain can be produced. It is a further object of the present invention to provide a cubic leucite-reinforced feldspathic dental porcelain composition which can be utilized in combination with low expansion materials in the fabrication of dental restorations.